Interconnecting communications networks

ABSTRACT

A digital cross-connection apparatus for use in interconnecting first and second communications network (STM-N) having respective first and second pluralities of data channels, wherein information data and communications management data of the channels in each network is multiplexed together, includes an add/drop unit (TTF, TSAF, LSC), connected to the first network (STM-N) and operable selectively to extract therefrom the data of a selected channel of the first plurality of channels, and a digital switching matrix (LPX) having an output connected to the second network (STM-n) and having an output connected to the add/drop unit (TTF, TSAF, LCS) for receiving therefrom the extracted data including management data from the first network. Thus, the management data can pass through the switching matrix (LPX) to the second network so that end-to-end path monitoring between the two networks is facilitated. The apparatus may be advantageously used in interconnecting two optical ring networks, for example, where, as in the case of Synchronous Digital Hierarchy (SDH) networks, it is desired to maintain path continuity from a node of the first network to a node of the second network.

TECHNICAL FIELD

The present invention relates to interconnecting communicationsnetworks, and in particular the present invention relates to digitalcross connect equipment.

BACKGROUND ART

The International Telegraph and Telephone Consultative Committee (CCITT)has established a set of recommendations describing the methodology fora recently-proposed digital transport network called Synchronous DigitalHierarchy (SDH).

These recommendations cover the transport frame structure, multiplexingmethods, basic outlines of the equipment functionality and the means ofmanaging this equipment. The directly relevant recommendations are:G.707, G.708, G.709, G781, G.782, G.783, G.784, G.773, G.sdx1, G.sdx2,G.sdx3, G.sna1, G.sna2, G.81s, and G.82j. Furthermore, therecommendations are backward compatible with the existing PDHrecommendations of G.702, and G.703, etc.

The CCITT recommendations are concerned with functionality and are notspecific to particular equipment implementation strategies. Therefore itis possible to combine several specific functional blocks to form aparticular equipment type.

The present invention can provide equipment conforming to the relevantstandards set out in the recommendations, and can assist in arriving atintegrated system solutions to equipment design problems in particularSDH applications.

In order to allow items of communications equipment on differentnetworks such as two optical ring networks to communicate with oneanother, conventionally a Digital Cross Connect element (DXC) isprovided. This DXC is essentially a digital switching matrix with anoperation interface for setting up relatively static connection betweeninput and output signals or channels.

In such a conventional cross connect, as defined by the CCITT, all theinterconnecting traffic from the ring networks needs to pass through thecross connect switch, and the data stream of each ring is demultiplexedinto its constituent channels and these channels are then all applied tothe switching matrix. Thus, even those channels of one ring that are notrequired to be routed through to the other ring pass through the matrix,and this gives rise to a number of problems as discussed briefly below.

Firstly, the switching capacity of the switching matrix must be largesince all of the ring channels must pass through it. Commonly only a fewthrough channels between two rings are actually required, so that theconventional DXC as outlined above is wasteful of hardware and henceexpensive to install. In addition, a major problem with rings of allkinds is that if the service demand grows unexpectedly on a portion ofthe ring the whole ring must be re-engineered and new capacityinstalled. Since the conventional DXC is already expensive andinherently wasteful of capacity it is not normal to provide sparecapacity, so that as the traffic on the ring increases the DXC requiresexpensive modification or even complete replacement.

The other problem is interconnection of several ring networks togetherwhile keeping the integrity of each ring unaffected. In this respect inthe conventional DXC the constituent channels (virtual containers--VCs)are effectively terminated in the cross connect and the path overhead ofthe constituent VC traffic passing through the DXC must be regenerated.This presents a problem in preserving the path continuity and pathmonitoring from end to end as is desired in all SDH networks.

DISCLOSURE OF THE INVENTION

From a close study of network applications for SDH equipments it has nowbeen realized that there is a need for a special type of equipment whichcan be successfully deployed at nodes where rings and mesh networks areinterconnected. Use of conventional SDH cross connect equipments inthese nodes was found to be inefficient and expensive.

An embodiment of the present invention employs the combination of thevery powerful and flexible functions of Add/Drop and Cross Connect toprovide a new combined functional element which will be referred tohereinafter as Add/Drop Cross Connect (ADX). In the relevant CCITTrecommendations the Add/Drop function is defined for Multiplexerequipments whereas the Cross Connect function has been introduced onlywithin the functional block description of Synchronous Digital CrossConnect equipments.

In the present application, in order to comply with the existing CCITTrecommendations, wherever possible, functional blocks from two mainareas of SDH technology (namely multiplexers and digital cross connects)have been used to express the concept behind the ADX.

Add/Drop functions are generally used conventionally in ring networkarchitectures whereas Cross Connect functions are more normally used inthe more complex mesh networks. Incidentally, the ability to use thering architecture for the transmission networks has only been madepossible by the recent adaption of SDH recommendations.

The ADX concept can combine the flexibility of cross connect equipmentswith the functionality of add/drop multiplexer equipment, when requiredas part of the same solution, and affords full expansion capabilities ofthe cross connect.

According to a first aspect of the present invention there is provideddigital cross-connection apparatus, for interconnecting first and secondcommunication networks having respective first and second pluralities ofdata channels, information data and communication management data of thechannels in each network being multiplexed together therein, whichapparatus comprises an add/drop unit, connected to the said firstnetwork and operable selectively to extract therefrom the data of aselected channel of the said first plurality, and a digital switchingmatrix having an output connected to the said second network and havingan input connected to the said add/drop unit for receiving therefromsuch extracted data including management data from the said firstnetwork, whereby such management data can pass through the saidswitching matrix to the said second network so that end-to-end pathmonitoring between the two networks is facilitated.

In such apparatus, the management information of the channels passingthrough the switching matrix is not stripped (disassembled) therefrom,so that it is not necessary to regenerate this information at the outputside of the switching means. This enables better path continuity from anode on one ring to a node on another ring to be maintained, therebyfacilitating reliable end-to-end path monitoring.

In addition, the particular combination of functional elements providessavings in circuitry and hence cost.

According to a second aspect of the present invention there is provideddigital cross connect apparatus for interconnecting first and secondcommunication networks carrying respectively first and second datastreams including first and second pluralities of data channels, whichapparatus includes:

first add/drop means for interposition in the first network;

second add/drop means for interposition in the second network; and

switching means connected between the said first and second add/dropmeans for passing data therebetween;

the said first add/drop means being operable to selectively drop fromthe first data stream the data of a preselected data channel of the saidfirst plurality, which dropped data is passed via the said switchingmeans to the said second add/drop means for addition thereby to the saidsecond data stream; and

the said second add/drop means being operable to selectively drop fromthe second data stream the data of a preselected data channel of thesaid second plurality, which dropped data is passed via the saidswitching means to the said first add/drop means for addition thereby tothe said first data stream;

the apparatus being such that management information relating to thedropped channels of the said first and second data streams is maintainedand passes through the said switching means, so that path continuity forsuch channels is preserved, whereby end-to-end path monitoring of eachnetwork is facilitated.

One of the major applications of ADX equipment embodying the presentinvention will be as a gateway node element for connection of trafficfrom several rings and also connection of transmission traffic to othernetwork elements.

One of the major advantages of ADX equipment embodying the presentinvention over conventional multiplexers or Cross Connect equipments isthat it is possible for the ring traffic integrity to be preserved andfor only the required selection of traffic channels to pass through thecross connect switch. This will have implications in simplifying themanagement and control of the network.

This clear differentiation of the local loop traffic from the ringinterconnection traffic is a major advantage of the ADX approach.

When interconnecting two rings, often not all (for example only 50%) ofthe traffic channels of one ring are required to be routed through tothe other ring. In a preferred embodiment of the present invention,because only the through channels need to be applied to the switchingmeans the capacity of those means can be smaller than in a conventionalDXC in which all of the channels of each ring are applied to the switch,i.e. local-only (non-through) channels are applied to the switchingmeans.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of an Add/Drop Cross Connect (ADX)embodying the present invention;

FIG. 2 is a more detailed functional block diagram corresponding to FIG.1;

FIG. 3A is a schematic diagram of a communication system including tworing networks interconnected by means of a conventional cross connect;

FIG. 3B is a schematic diagram corresponding to FIG. 3A, but in whichthe two rings are interconnected by means of an Add/Drop Cross Connectembodying the present invention;

FIG. 4A is detailed block diagram of the conventional cross connect ofFIG. 3A;

FIG. 4B is a detailed block diagram of the Add/Drop Cross Connect ofFIG. 3B;

FIG. 5A is a diagram for illustrating an example of the operation of theconventional cross connect of FIG. 3A;

FIG. 5B is a diagram for illustrating a corresponding example of theoperation of the Add/Drop cross connect of FIG. 3B;

FIG. 6 is a functional block diagram of a conventional cross connect(for comparison with FIG. 1);

FIG. 7 is a more detailed functional block diagram corresponding to FIG.6 (for comparison with FIG. 2);

FIGS. 8A to 8D, 9A to 9C, and 10A to 10C are block diagrams illustratingthe manner in which Add/Drop Cross Connect equipment embodying thepresent invention can be used in conjunction with SDH line transmissionequipment;

FIGS. 11A to 12C are block diagrams illustrating the manner in whichAdd/Drop Cross Connect equipment embodying the present invention can beused in conjunction with other examples of SDH line transmissionequipment;

FIG. 13 is a schematic diagram illustrating use of ADX equipmentembodying the present invention as a gateway node for local, regionaland national network traffic;

FIG. 14 is a schematic diagram illustrating an example of the use of ADXequipment embodying the present invention in interconnecting ringnetworks and other network elements; and

FIG. 15 is a schematic diagram illustrating an example of the use of ADXequipment embodying the present invention for the interconnection ofthree ring networks to central office switching equipments;

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 illustrates the proposed functional architecture for ADXequipment 10 embodying the present invention, wherein the switching unitLPX (Lower Order Path Cross Connection) 12 acts as the central unit forswitching of the transmission traffic channels to the requireddirection.

The virtual container (VC) paths are interconnected through the crossconnect of FIG. 1 without needing to terminate and regenerate the pathoverhead of the constituent VC traffic passing through the ADX. This isimportant in preserving the path continuity and path monitoring from endto end as is desired in all SDH networks.

The Time Slot Assignment Function (TSAF) 14 enables selection ofappropriate channels for the SDH frame for cross connection in the LPX12 without needing to demultiplex and terminate all the incoming trafficchannels, as in the case with a conventional cross connect (DXC). Moreefficient use of the switch facility is made since only the paths thatare cross connected between the different networks are passed throughthe LPX 12.

As defined in the CCITT 1988 recommendations G.781, G.782, G.783 andG.sdxc-3, Overhead Access Function (OHA) 16 provides access in anintegrated manner to the transmission overhead functions wherenecessary. Message Communication Function (MCF) 18 receives and buffersmessages from the Data Communication Channels (DCCs), Q-and F-interfacesand adx (SEME) 20. ADX Synchronous Equipment Management Function(adxSEMF) 20 converts performance data and implementation specifichardware alarms into object orientated messages for transmission on theDCCs and or a Q interface. It also converts object oriented messagesrelated to other management functions for passing across the Snreference points. ADX Timing Source function (adx TS) 22 provides timingreference to appropriate functional blocks as indicated in FIG. 1. Thisfunction includes an internal oscillator function and ADX timinggenerator function. ADX Timing Physical Interface (adx TPI) 24 providesthe interface between the external synchronization signal and the adxTS22 and should have the physical characteristics of one of the G.703synchronization interfaces. The other function blocks are explained withreference to FIG. 2.

FIG. 2 illustrates the constituent functional blocks within the compoundfunctional elements (TTF, etc.) of the ADX 10 shown in FIG. 1.Definition of the individual functional blocks illustrated in FIG. 2 canbe found in CCITT recommendation G.782, and G.783.

The Transport Terminal Function (TTF) 26 includes an SDH PhysicalInterface (SPI) 28, a Regenerator Section Termination (RST) 30, aMultiplex Section Termination (MST) 32, a Multiplex Section Protection(MSP) 34 and a Section Adaptation (SA) 36. As defined in CCITTrecommendations, the SPI 28 converts an internal logic level STM(Synchronous Transport Module)-N (N=1, 4, 16, etc.) signal into an STM-Nline interface signal, and the RST 30 generates a Regenerator SectionOverhead (RSOH) comprising rows 1 to 3 of a Section Overhead (SOH) ofthe STM-N signal in the process of forming an SDH frame signal andterminates the RSOH in the reverse direction. The MST 32 generates aMultiplex Section Overhead (MSOH) comprising rows 5 to 9 of the SOH ofthe STM-N signal in the process of forming an SDH frame signal andterminates the MSOH in the reverse direction. The MSP 34 providescapability for switching a signal between and including two MSTfunctions, from a `working` to a `protection` section. The SA 36 processan AU-3/4 pointer to indicate the phase of the VC-3/4 path Overhead(POH) relative to the STM-N SOH and assembles/diassembles the completeSTM-N frame.

The TSAF 14 includes a Higher order Path Connection (HPC) 38, a Higherorder Path Termination (HPT) 40, Higher order Path Adaptation (HPA) 42,and Lower order Path Connection (LPC) 44. As defined in CCITTrecommendations, the HPC 38 provides for flexible interconnection ofhigher order VCs (VC-3/4). The HPT 40 terminates a higher order path bygenerating and adding the appropriate VC POH to the relevant containerat the path source and removing the VC POH and reading it at the pathsink. The HPA 42 adapts a lower order VC (VC-1/2/3) to a higher order VC(VC-3/4) by processing the TU pointer which indicates the phase of theVC-1/2/3 POH relative to the VC-3/4 POH and assembling/disassembling thecomplete VC-3/4. The LPC 44 allows flexible interconnection of the lowerorder VCs (VC-1/2/3).

The Higher order Assembler (HA) 46 includes the HPT 40 and the HPA 42.

The Higher order Interface (HI) 48 includes a Physical Interface (PI)50, a Lower order Path Adaptation (LPA) 52 and the HPT 40. The LPA 52adapts a PDH (Plesiochronous Digital Hierarchy) signal to an SDH networkby mapping/demapping the signal into/out of a synchronous container. Ifthe signal is asynchronous, the mapping process will include bit leveljustification.

The Lower order Interface (LI) 54 includes the PI 50, the LPA 52 and aLower order Path Termination (LPT) 56.

The LPT 56 terminates a lower order path by generating and adding theappropriate VC POH to the relevant container at the path source andremoving the VC POH and reading it at the path sink.

The Lower order Connection Supervision (LCS) 58 enables supervision ofthe unassigned and assigned Lower Order connections. Since it hasidentical information flow across it's input and outputs it may beoptional or degenerate. (LCS acts as a source and sink for parts of thelower order path overhead.)

FIGS. 3A and 3B illustrate a comparative example for explainingdifferences between the conventional Cross Connect DXC approach and thenew Add/Drop Cross Connect ADX approach. FIG. 3A illustrates aconventional method of interconnecting two ring networks 60, 62 such asoptical ring networks. Each loop 60, 62 has connected thereto aplurality of items of communications equipment (not shown), each ofwhich is linked to its ring by an Add/Drop Multiplexer (ADM) 64. TheseAdd/Drop Multiplexers 64 allow the items of equipment to receive (drop)data from a particular channel of the data stream carried by the ring(for example at 140 MHz) or to transmit (add) data on a particularchannel.

In FIG. 3A all the ring traffic from loop 60 and loop 62 passes throughSwitch Matrix unit 68 of the Cross Connect 66 but in FIG. 3B only therequired channels are selected by the Add/Drop unit 70 (including twoTTFs 26 and a TSAF 14 shown in FIG. 1) to pass from loop 60 to loop 62using the Switch Matrix unit 74 of the ADX 72. This has advantages inincreasing the effective capacity of the Cross Connect and insimplifying control and management issues.

FIG. 4 is an expansion of the concept illustrated in FIG. 3. It is asimplified illustration of the comparison between the operation of theconventional DXC and the ADX for one level of the SDH hierarchy.

In this example we require to cross connect VC-3's (i.e. virtualcontainers of hierarchy level 3) from two STM-1 (Synchronous TransportModule type 1) rings. In FIG. 4A all the traffic from the STM-1 lines isdemultiplexed to VC-3 level through TTF 26, HA 46 and LCS 58 andswitched through to the appropriate output ports of the LPX (Lower OrderPath Cross Connection) 12.

It can be seen that in this example 6 inputs and 6 outputs of the LPX 12are occupied.

In FIG. 4B the traffic from STM-1 lines passes through TTF 26 and theTSAF 14 which selects only the appropriate VC-3 time slots of the STM-1signals for cross connection through the LPX 12. It can be seen that inthis particular example only 2 input and 2 output ports of the LPX 12are occupied.

It should be noted that the advantage of ADX disappears if all thetraffic channels (VC-3's) from the STM-1 lines are to be crossconnected, in which case the total number of the LPX ports occupied forthe ADX are the same as that of the DXC.

FIG. 5 further illustrates the same principle as FIG. 3. In FIG. 5A theSTM frame 76 is processed by the TTF (Transport Terminal Function) 26which provides access to the management information contained in the DCC(data communications channel) included in the STM frame overheads. Afterappropriate pointer processing by the HA (Higher Order Assembler) 46 theVC-3's are passed to the LPX 12 of the conventional DXC which carriesout the switching function on all the input VC-3's and the output fromthe LPX 12 is multiplexed up to a VC-4 in the HA (Higher Order Assemblerfunction) 46 and passed to the TTF function 26 for insertion of theappropriate path overhead and management information.

In FIG. 5B, on the other hand, the STM frame is processed by the TTF(transport terminal function) 26 which provides access to the managementinformation contained in the DCC (data communications channel) includedin the STM frame overheads. After pointer processing by the TSAF (TimeSlot Assignment function) 14 the appropriate (shaded) VC-3 is passed tothe LPX 12 of the ADX which carries out the switching function betweenthe two VC-3's to be cross connected between the STM-1 frames. Therespective VC-3's that have passed through the LPX 12 are then assembledin the appropriate output STM frame by the TSAF's 14 and sent to theTTF's 26 for insertion of the appropriate overhead and managementinformation.

Incidentally, although it might initially be thought that a similarfunction to the ADX function might be achieved by direct connection ofan Add/Drop Multiplexer ADM and a conventional DXC, there are practicaldifficulties in such direct connection such as the loss of path overheadand path continuity when interconnecting transmission traffic containedin the virtual containers. This arises, for example when interconnectingVC-12's containing 2 Mbit/s payloads from an ADM to a DXC, because thepath overheads on the individual VC-12's are terminated by themultiplexer and are again regenerated by the DXC. This leads to the lossof path continuity needed for the end-to-end path monitoring carried outin all SDH networks. Also, in going from an ADM to a DXC certainfunctions are duplicated and this leads to an inefficient and expensivesystem. In the proposed ADX equipment embodying the present inventionpath continuity for the individual VC's is preserved and duplication offunctions is avoided with the signal only going through the necessaryprocesses. Functions such as Higher Order Path Termination (HPT), LowerOrder Path Termination (LPT), Lower Order Path Adaptation (LPA),Physical Interface (PI) etc., which are normally used in an ADM, areavoided in the ADX architecture.

Furthermore, somewhat surprisingly it is found that although the ADXarchitecture has been generated from some of the functional blocks usedin Add/Drop multiplexers and Digital Cross Connect equipment, ADXequipment embodying the invention offers more (in terms of function andflexiblity) than the mere sum of its constituent functional elementscombined in conventional manner. This is because, as noted above, theADX approach leads to savings in terms of hardware requirements and inthe simplification of the management and control of the transmissionnetwork traffic.

For comparison with the functional architecture of the ADX equipmentaccording to the present invention, the functional architecture of asynchronous Digital Cross Connect (DXC) shown in the CCITTrecommendations and the constituent functional block within the compoundfunctional elements are shown in FIG. 6 and 7, respectively. As shown inFIGS. 6 and 7, all virtual containers from the STM-N are applied to theLPX in the DXC.

As shown in FIG. 1, besides branches of add/drop units each includingtwo TTF 26 and a TSAF 14, three branches 80, 82 and 84 for connectionwith STM-N, a higher order signal and a lower order signal of G.703 areprovided for the LPX 12 of the ADX equipment according to the presentinvention. As shown in FIGS. 8 to 10, the ADX equipment of the presentinvention can inplement SDH elements such as a terminal multiplexer, anadd/drop multiplexer, an add/drop cross connect by utilizing these threebranches and in conjunction with other line transmission equipment suchas FLM 2400E, FLM 600E, and FLM 150E.

FIG. 8 illustrates some application examples of how ADX equipmentembodying the invention (ADX 4/1) can be used in conjunction withFujitsu's FLM 2400E and FLM 600E range of line transmission equipmentsfor SDH.

FIG. 8A represents application of the ADX as a Terminal Multiplexer inwhich 16×STM-1 lines from FLM 2400E TRM unit are demultiplexed down toprimary rate of 2 MBit/s.

FIG. 8B represent application of the ADX as an Add/Drop Multiplexer inwhich 16×STM-1 lines from FLM 2400E ADM unit are demultiplexed down toprimary rate of 2 MBit/s. In this application any of the 2 MBit/schannels within the main STM-16 line (Ring) can be accessed through theADX.

FIG. 8C represents application of the ADX as an Add/Drop Cross Connectin which 8×STM-1 lines from each of the FLM 2400E ADM units aredemultiplexed down to primary rate of 2 MBit/s, and any of the 2 MBit/schannels within the 8×STM-1 frames of the main STM-16 lines (Rings) canbe accessed through the ADX.

FIG. 8D represents application of the ADX as a Cross Connect in which8×STM-1 lines from the FLM 2400E ADM unit are demultiplexed down to aprimary rate of 2 MBit/s, and any of the 2 MBit/s channels within8×STM-1 frames of the main STM-16 line (Rings) can be accessed throughthe ADX. The traffic from the FLM 2400E ADM can also be interconnectedto selected traffic channels from the two FLM 600E TRM and ADM units.

FIG. 9 illustrates some application examples of how ADX equipmentembodying the invention (ADX 4/1) can be used in conjunction withFujitsu's FLM 600E and FLM 150E range of line transmission equipmentsfor SDH.

FIG. 9A represents application of the ADX as a Terminal Multiplexer inwhich 4×STM-1 lines from four FLM 600E TRM units are demultiplexed downto primary rate of 2 MBit/s.

FIG. 9B represents application of the ADX as an Add/Drop Cross Connectin which 4×STM-1 lines from FLM 2400E ADM units are demultiplexed downto primary rate of 2 MBit/s. In this application any of the 2 MBit/schannels within the main STM-4 lines (Rings) can be accessed through theADX.

FIG. 9C represents application of the ADX as a Cross Connect in which4×STM-1 lines from each of the FLM 600E units are demultiplexed down toprimary rate of 2 MBit/s, and any of the 2 MBit/s channels within the4×STM-1 frames of the main STM-4 lines can be accessed through the ADX.The traffic from the FLM 600E units can also be cross connected to 8STM-1 lines (rings) connected to the ADX through FLM 150E's using 2MBit/s interfaces.

FIG. 10 illustrates some application examples of how the ADX 4/1 can beused in conjunction with Fujitsu's FLM 150E range of line transmissionequipments for SDH.

FIG. 10A represents application of the ADX as a Terminal Multiplexer inwhich 16×STM-1 lines from sixteen FLM 150E TRM units are demultiplexeddown to primary rate of 2 MBit/s.

FIG. 10B represents application of the ADX as an Add/Drop Cross Connectin which 16×STM-1 lines from FLM 150E ADM units are demultiplexed downto primary rate of 2 MBit/s. In this application any of the 2 MBit/schannels within the main STM-1 lines (rings) can be accessed through theADX.

FIG. 10C represents application of the ADX as a Cross Connect in which16×STM-1 lines from each of the FLM 150E ADMs are demultiplexed down toprimary rate of 2 MBit/s, and any of the 2 MBit/s channels within theSTM-1 frames of the main STM-1 lines can be accessed through the ADX.The traffic from the FLM 150E units can also be cross connected between32 STM-1 lines (rings) connected to the ADX through FLM 150E's using 2MBit/s interfaces.

In FIGS. 8, 9, and 10 so far described only STM-1 TRM and 2 MBit/sinterface units of the ADX were used. In the following examples furtherunits are introduced. These are STM-1 AD, STM-4 TRM and STM-4 ADinterface units. This is more integrated systems approached and it canbe seen that many of the previous functions can be carried out withoutthe use of the FLM range of equipments.

FIG. 11 illustrates some application examples of how the ADX 4/1 can beused with STM-1 AD, STM-4 TRM and STM-4 AD interface units.

FIG. 11A represents application of the ADX as a Terminal Multiplexer inwhich 4×STM-1 lines are demultiplexed down to primary rate of 2 MBit/s.

FIG. 11B represents application of the ADX as an Add/Drop cross connectin which 4×STM-4 lines are demultiplexed down to primary rate of 2MBit/s. In this application any of the 2 MBit/s channels within the mainSTM-4 lines (rings) can be accessed through the ADX.

FIG. 11C represents application of the ADX as a Cross Connect in which4×STM-4 lines are demultiplexed down to primary rate of 2 MBit/s, andany of the 2 MBit/s channels within the 4×STM-1 frames of the main STM-4lines can be accessed through the ADX. The traffic from the STM-4 linescan also be cross connected to 8 STM-1 lines (rings) connected to theADX through the STM-1 AD interfaces.

FIG. 12 illustrates some application examples of how the ADX 4/1 can beused in conjunction with Fujitsu's FLM 150E range of line transmissionequipments for SDH.

FIG. 12A represents application of the ADX as a Terminal Multiplexer inwhich 16×STM-1 lines from sixteen FLM 150E TRM units are demultiplexeddown to primary rate of 2 MBit/s.

FIG. 12B represents application of the ADX as an Add/Drop Cross Connectin which 16×STM-1 lines from the FLM 150E ADM units are demultiplexeddown to primary rate of 2 MBit/s. In this application any of the 2MBit/s channels within the main STM-1 line (rings) can be accessedthrough the ADX.

FIG. 12C represents application of the ADX as a Cross Connect in which16×STM-1 lines from each of the FLM 150E ADMs are demultiplexed down toprimary rate of 2 MBit/s, and any of the 2 MBit/s channels within theSTM-1 frames of the main STM-1 lines can be accessed through the ADX.The traffic from the FLM 150E units can be cross connected between 32STM-1 lines (rings) connected to the ADX through the FLM 150E's using 2MBit/s interfaces.

It is important to note that while the drawings refer to a certain sizeof cross connect switch this is of no consequence to the real conceptbehind ADX and this equipment might be produced with varying sizes. Theequipment is upgradeable according to network application requirements.

Furthermore, the equipment has the ability to handle the followinginterfaces:

1.5 MBit/s, 2 MBit/s, 34 MBit/s, 45 MBit/s interfaces as defined byCCITT recommendations.

140 MBit/s PDH Interfaces

STM-1 with electrical and optical interfaces

STM-4 with optical interfaces.

FIG. 13 illustrates the application of the ADX as a gateway node forlocal, regional and national network traffic. In this example trafficfrom the trunk national network is accessed via an FLM 2400E ADM.8×STM-1 channels from the main STM-16 line (ring) are demultiplexed downto STM-1 tributaries which in turn are then connected to the ADX via 8STM-1 Tributary Terminal units. The STM-1 tributaries are thendemultiplexed down to the primary rate and appropriate paths set up toany of the ADX Tributary units serving the regional and local networksand vice versa. Transmission traffic from the local network can bedirected to other tributaries in the local network or to regional ornational network and vice versa.

FIG. 14 shows an example for the use of ADX in interconnection of ringnetwork traffic and also connection to other network elements. In thisexample an ADX connects two STM-4 ring traffics to each other and to acentral office switch. Other ADX equipments are used at different nodesas illustrated for flexible interconnection of traffic from the STM-1local loop, and other network elements as illustrated.

FIG. 15 illustrates the application of ADX for connection of three ringnetworks to central office switching equipments and to each other usingSTM-1 and STM-4 Add/Drop interfaces. The reference numeral 80 denotesthe FLM 150E Add/Drop Multiplexer, the reference numeral 82 denotes anSTM-4 Add/Drop Optical unit, the reference numeral 84 denotes an STM-1Add/Drop Optical unit, and the reference numeral 86 denotes a 2 MBit/sG.703 Interface unit. The traffic collected by loop 88 and loop 90 canbe connected to the central switching equipment at any of the nodes 92,94, or 96. It can also be connected to any other node as a leased linecircuit.

I claim:
 1. Digital cross-connection apparatus, for interconnectingfirst and second communications networks having respective first andsecond pluralities of data channels, information data and communicationsmanagement data of the channels in each network being multiplexedtogether therein, which apparatus comprises:an add/drop unit, connectedto said first communications network and operable selectively to extracttherefrom data of a selected channel of said first plurality, and adigital switching matrix having an output connected to said secondcommunications network and having an input connected to said add/dropunit for receiving therefrom extracted data including management datafrom said first communications network, said extracted management datapassing through said switching matrix to said second communicationnetwork so that end-to-end path monitoring between the two networks isfacilitated.
 2. Digital cross connect apparatus for interconnectingfirst and second communications networks carrying respectively first andsecond data streams including first and second pluralities of datachannels, which apparatus comprises:first add/drop means forinterposition in the first communications network; second add/drop meansfor interposition in the second communications network; and switchingmeans connected between said first and second add/drop means for passingdata therebetween; said first add/drop means being operable toselectively drop from the first data stream data of a preselected datachannel of said first plurality, which dropped data is passed via saidswitching means to said second add/drop means for addition thereby tosaid second data stream; and said second add/drop means being operableto selectively drop from the second data stream data of a preselecteddata channel of said second plurality, which dropped data is passed viasaid switching means to said first add/drop means for addition therebyto said first data stream; the apparatus being such that managementinformation relating to the dropped channels of said first and seconddata streams is maintained and passes through said switching means, sothat path continuity for said channels is preserved, whereby end-to-endpath monitoring of each network is facilitated.
 3. Digital cross connectapparatus for interconnecting respective first and second communicationnetworks, comprising:first interface means for communication to saidfirst communication network to receive a data stream carried thereby,which data stream includes respective first data items correspondingrespectively to a first plurality of data channels, and operable toprocess the received data stream to derive therefrom said respectivefirst data items; first time slot assignment means connected to saidfirst interface means for receiving therefrom each of said first dataitems and having storage means for storing a set of the received firstdata items, said first time slot assignment means being operableselectively to pass to an output port thereof a preselected one of thestored first data items which is to be transmitted to said secondcommunication network, and operable also to receive at an input portthereof a replacement first data item and to store said replacementfirst data item in said storage means in place of said preselected firstdata item; second interface means connected to said first time slotassignment means for receiving therefrom the stored set of first dataitems and operable to process said first data items to form a modifieddata stream for application to said first communication network; thirdinterface means for connection to said second communication network toreceive a data stream carried thereby, which data stream includesrespective second data items corresponding respectively to a secondplurality of data channels, and operable to process the received datastream to derive therefrom said respective second data items; secondtime slot assignment means connected to said third interface means forreceiving therefrom each of said second data items and having storagemeans for storing a set of the received second data items, said secondtime slot assignment means being operable selectively to pass to anoutput port thereof a preselected one of the stored second data itemswhich is to be transmitted to said first communication network, andoperable also to receive at an input port thereof a replacement seconddata item and to store said replacement second data item in said storagemeans in place of the preselected second data item; fourth interfacemeans connected to said second time slot assignment means for receivingtherefrom the stored set of second data items and operable to processsaid second data items to form a modified data stream for application tosaid second communication network; and switching means connected to saidfirst and second time slot assignment means for selectively establishinga connection between said output port of said first time slot assignmentmeans and said input port of said second time slot assignment means andbetween said output port of said second time slot assignment means andsaid input port of said first time slot assignment means, thereby topermit passage, via said switching means, of the preselected first andsecond data items, including management information thereof, between thetwo time slot assignment means to serve respectively as said replacementsecond and first data items.
 4. An add/drop cross connect apparatus forinterconnecting first and second communication path each transportingvirtual containers containing information data and path overheads formonitoring transportation of the corresponding virtual containers,comprising:first add/drop means for adding and dropping preselectedvirtual containers to and from the first communication path,respectively, without disassembling the added virtual containers and thedropped virtual containers; second add/drop means for adding anddropping preselected virtual containers to and from the secondcommunication path, respectively, without disassembling the addedvirtual containers and the dropped virtual containers; and crossconnection means for switching the virtual containers dropped by thefirst and the second add/drop means and the virtual containers to beadded by the first and the second add/drop means, to thereby allow crossconnection between preselected parts of the first and the secondcommunication paths.
 5. An add/drop cross connect apparatus as claimedin claim 4, further comprising:first interface means for providing aninterface between the cross connect means and a third communication pathtransporting information data not accompanied by the path overhead, tothereby allow cross-connection between the third communication path andother communication path associated with the cross connect means; andsecond interface means for providing an interface between the crossconnect means and a fourth communication path transporting virtualcontainers containing information data and the path overheads, tothereby allow cross-connection between the fourth communication path andanother communication path associated with the cross connect means. 6.An add/drop cross connect apparatus as claimed in claim 4, wherein thefirst add/drop means includes:first transport terminal means, connectedto one end of the first communication path, for terminating the firstcommunication path and for receiving and sending the virtual containers;second transport terminal means, connected to other end of the firstcommunication path, for terminating the first communication path and forreceiving and sending the virtual containers; and first time slotassignment means, connected to the first and the second transportterminal means and to the cross connect means, for connecting the firstand the second transport terminal means with each other, for adding anddropping the preselected virtual containers between the first and secondtransport terminal means, for receiving the virtual containers to beadded from the cross connect means, and for sending the dropped virtualcontainers to the cross connect means, and wherein the second and/dropmeans includes: third transport terminal means connected to one end ofthe second communication path, for terminating the second communicationpath and for receiving and sending the virtual containers; fourthtransport terminal means, connected to other end of the secondcommunication path and for receiving and sending the virtual containers;and second time slot assignment means, connected to the third and thefourth transport terminal means and to the cross connect means, forconnecting the third and the fourth transport terminal means with eachother, for adding and dropping the preselected virtual containersbetween the third and fourth transport terminal means, for receiving thevirtual containers to be added from the cross connect means, and forsending the dropped virtual containers to the cross connect means.
 7. Anadd/drop cross connect apparatus as claimed in claim 6, wherein thefirst time slot assignment means include a first path connection unit,connected to the first and the second transport terminal means, forproviding flexible interconnection among the virtual containerstransported on the first communication path, the added virtualcontainers and the dropped virtual containers, andwherein the secondtime slot assignment means include a second path connection unit,connected to the third and the fourth transport terminal means, forproviding flexible interconnection among the virtual containerstransported on the second communication path, the added virtualcontainers and the dropped virtual containers.